1. Field of the Invention
The present invention relates to methods of, and apparatus for, measuring the degradation of a data stream. It has particular utility in relation to providing a measure for use in selecting a speech coding rate for use over a radio communication link between a mobile telephone and a fixed network.
2. Description of Related Art
Conventionally, degradation of a data stream has been estimated on the basis of a measurable parameter of the channel over which the data stream has been sent. For example, the degradation of coded speech carried over a radio channel has hitherto been estimated on the basis of a measured radio channel parameter such as the Carrier to Interferer (C/I) ratio or the Bit Error Rate. In practice, it is found that such estimates are unreliable indicators of the degree of degradation experienced by the data stream.
European patent application no. 0 798 888 discloses a speech encoder/decoder for use in a mobile telephone. The encoder divides the incoming speech into frames and derives a set of speech parameters for each frame. The decoder is operable to receive decoded speech parameters for a frame and to determine whether they contain errors that are so significant that they are unsuitable for generating synthesised speech. In making the determination, the encoder arranges the bits it outputs according to their importance to the quality of the decoded speech. Each bit is associated with an importance class which is in turn associated with a importance value. A checksum is applied to each class of bits at the encoder. The decoder analyses the checksum and thereby indicates whether the associated class contains any errors. The importance values for those classes which contain errors are then added together and their sum is compared to a threshold. If the sum exceeds the threshold then the decoded parameters are deemed unsuitable for generating synthesised speech.
EP 0 798 888 discloses a measure of the degradation of one section of a data stream. When measuring the degradation of a data stream comprising a plurality of sections, it is not obvious that the measure for one section is any better than the conventional measurements of Bit Error Rate.
A first aspect of the present invention provides a method of measuring the degradation caused by the influence of errors on a data stream comprising a plurality of sections, said method comprising the steps of:
retrieving stored error significance values associated with respective error significance classes of one or more digits of a section of said data stream;
determining error likelihood measures for respective error likelihood classes of one or more digits in said section;
calculating a degradation measure on the basis of the sum over said section of said error likelihood measure for each digit weighted by the error significance value associated with that digit.
The present inventor realised that such a degradation measure (i.e. one based on a sum over a section of an error likelihood estimate for each digit weighted by an error significance value for each digit) is a better measure of the degradation of a data stream than a Bit Error Rate or the like. This is because many coding schemes are designed to give an inverse correlation between the likelihood of a received digit being in error and the significance of an error in that digit. Where there is such a correlation, the coding scheme will operate to reduce the impact of an increase in Bit Error Rate. The measure according to the present invention takes the effect of such coding schemes into account.
It is anticipated that, in the majority of embodiments of the present invention, said digits will be bits (binary digits). The invention might also be applied to other forms of coding such as quaternary or octal coding.
In some embodiments, said error significance classes comprise bits at predetermined positions in the sections, and said error significance values retrieving step involves determining the class to which a bit belongs on the basis of the position of said bit within the section.
Some channel encoding/decoding algorithms provide a measure indicative of the likelihood of error in each decoded bit. Hence, in some embodiments said error likelihood measures are provided on the basis of an error likelihood measure provided for each bit by a channel decoding method. This has the advantage that the calculation of the degradation measure can be implemented without increasing the bit-rate of the transmitted signal.
As stated above, one application of the present invention is in the selection of a coding scheme to be used over a communication link between a sender which is operable to encode a signal using a selected one of a number of possible coding methods and a receiver. Therefore, according to a second aspect of the present invention, there is provided a method of selecting a signal encoding method for use over a communications link, said method comprising:
carrying out a method according to the first aspect of the present invention to determine said degradation measure on the basis of a received portion of said data stream; and
selecting one of a plurality of encoding methods on the basis of said degradation measure.
According to a third aspect of the present invention, there is provided a method of measuring the degradation of a section of a data stream owing to the influence of errors thereon, said method comprising the steps of:
retrieving stored error significance values associated with respective error significance classes of one or more digits of said section; said method being characterised by;
determining error likelihood estimates for respective error likelihood classes of one or more digits in said section;
calculating a measure of said degradation on the basis of the sum over said digits of said error likelihood estimate for each digit weighted by the error significance value associated with that digit.
By determining an estimate of the likelihood of each digit of the section being in error, a more precise measure of the degradation suffered by a section of the data stream transmitted across a communication link is obtained. This is useful in situations where it is important to determine more precisely the extent to which a group of digits has been corrupted. The term estimate is intended to exclude variables that have only two possible values.